Road Map CS Concepts Data Structures Java Language Java Collections Client/Implementer Efficiency Recursion Regular Expressions Grammars Sorting Backtracking Hashing Huffman Compression Data Structures Lists Stacks Queues Sets Maps Priority Queues Java Language Exceptions Interfaces References Comparable Generics Inheritance/Polymorphism Abstract Classes Java Collections Arrays ArrayList 🛠 LinkedList 🛠 Stack TreeSet / TreeMap HashSet / HashMap PriorityQueue

Exercise Write a program that counts the number of unique words in a large text file (say, Moby Dick or the King James Bible). Store the words in a collection and report the # of unique words. Once you've created this collection, allow the user to search it to see whether various words appear in the text file. What collection is appropriate for this problem?

Sets (11.2) set: A collection of unique values (no duplicates allowed) that can perform the following operations efficiently: add, remove, search (contains) We don't think of a set as having indexes; we just add things to the set in general and don't worry about order set.contains("to") true set "the" "of" "from" "to" "she" "you" "him" "why" "in" "down" "by" "if" set.contains("be") false

Set implementation in Java, sets are represented by Set type in java.util Set is implemented by HashSet and TreeSet classes TreeSet: implemented using a "binary search tree"; pretty fast: O(log N) for all operations elements are stored in sorted order HashSet: implemented using a "hash table" array; very fast: O(1) for all operations elements are stored in unpredictable order

Counting What if we wanted to use something other than an int as an index? count digits: 22092310907 // (C)hocolate, (V)anilla, (S)trawberry count votes: ”CVVVVVVCCCCCVVVVVVCVCCSCVCCSCVCCSV" index 1 2 3 4 5 6 7 8 9 value key ”C" ”V" "S" value 16 14 3

Maps (11.3) map: Holds a set of unique keys and a collection of values, where each key is associated with one value. a.k.a. "dictionary", "associative array", "hash" basic map operations: put(key, value ): Adds a mapping from a key to a value. get(key ): Retrieves the value mapped to the key. remove(key ): Removes the given key and its mapped value. myMap.get("Aug") returns 37.3

Maps (11.3) map: Holds a set of key-value pairs, where each key is unique a.k.a. "dictionary", "associative array", "hash" map.get("the") 56 set key value "at" 43 key value "you" 22 key value "in" 37 key value "why" 14 key value "me" 31 key value "the" 56

Map implementation in Java, maps are represented by Map type in java.util Map is implemented by the HashMap and TreeMap classes TreeMap: implemented as a linked "binary tree" structure; very fast: O(log N) ; keys are stored in sorted order HashMap: implemented using an array called a "hash table"; extremely fast: O(1) ; keys are stored in unpredictable order A map requires 2 type params: one for keys, one for values. // maps from String keys to Integer values Map<String, Integer> votes = new HashMap<String, Integer>();

Map methods put(key, value) adds a mapping from the given key to the given value; if the key already exists, replaces its value with the given one get(key) returns the value mapped to the given key (null if not found) containsKey(key) returns true if the map contains a mapping for the given key remove(key) removes any existing mapping for the given key clear() removes all key/value pairs from the map size() returns the number of key/value pairs in the map isEmpty() returns true if the map's size is 0 toString() returns a string such as "{a=90, d=60, c=70}" keySet() returns a set of all keys in the map values() returns a collection of all values in the map putAll(map) adds all key/value pairs from the given map to this map equals(map) returns true if given map has the same mappings as this one